I was slightly underhand about the way I did it, I knew where the cop bar in my local city was so I wandered in there for a drink and got talking to the barman. Asked him if he knew anyone who might want to help and he ended up calling someone over and telling them I'd buy them a drink if they spoke to me about police procedure for my book. It was kind of strange - but that officer did tell me that they have special community relations officers in most headquarters who deal with this kind of situation though this would be in Ireland and he mentioned they have this in the UK too. So long as your book isn't bashing the police they're happy for the publicity.
That being said, took me a long time to work up the courage to actually call even though I felt silly about being nervous after
